1. Field of the Invention
The present invention relates to a distortion generating circuit which provides distortion for canceling nonlinearity of input/output characteristics of a high-frequency signal output stage (for example a nonlinear device such as a nonlinear amplifier as an output stage in transmitter or a semiconductor laser) to linearize an analog signal output therefrom. More particularly, the invention relates to a distortion generating circuit suitable for multiple analog image transmission such as CATV.
2. Related Background Art
Conventionally, negative feedback is used to improve the linearity of an amplifier in the low frequency region. However, conventional systems do not normally use the negative feedback to improve the linearity of the amplifier in the high frequency region because of rotation of phase caused by a signal delay. Thus, when high linearity is required in a broad region, the linearity is normally improved using a circuit having input/output characteristics which cancel the nonlinearity (i.e., a distortion generating circuit). A distortion generating circuit is provided before an output stage or a semiconductor laser of a transmitter. It has a nonlinear element such as a diode or a transistor. Based on the circuit's nonlinearity, distortion is generated, that distortion being used to linearize an object circuit having nonlinearity. Specific examples are described for example in Japanese Utility Model Publication No. 2-30192 and ELECTRONICS LETTERS Vol. 28 No. 20, 1992, pp 1875-1876. The circuits as described in these references are arranged with two diodes, as nonlinear elements, which are connected opposite to each other with respect to a signal and which are operated as nonlinear elements at a predetermined bias point.
The multiple analog image transmission such as the CATV requires transfer characteristics of very low level distortion to obtain a high quality image. The distortion correcting circuit in multiple analog image transmission needs to have the following characteristics: (1) Independent correction of second order and third-order distortions components, which could cause a problem in analog image transmission; and (2) correction of the distortion while also compensating a frequency dependency of a nonlinear device, because wide band transmission is required. There are distortion correcting circuits developed for correcting the nonlinearity of a transmission device, which could be a cause to produce distortion in analog signal. For example, U.S. Pat. No. 4,992,754, U.S. Pat. No. 5,132,639 and U.S. Pat. No. 5,119,392 describe such circuits.
FIG. 1 diagrammatically shows a circuit as described in U.S. Pat. No. 4,992,754 and U.S. Pat. No. 5,132,639. This circuit is constructed in a so-called parallel circuit arrangement, where a signal to be supplied to a circuit 901 is split by a distributer 910 into two paths. A nonlinear device 915 is provided in one path, and a delay circuit (delay line) 925 is provided in the other path. Furthermore, two signals are combined in a coupler 911. This circuit has such an advantage that distortion components can have frequency characteristics without influencing a base signal and the frequency dependency of distortion of the nonlinear device can also be compensated.